Here are some hints to get you started:
1. Ignore the width of the table- (i.e. assume that the probability of the pencil landing on the table is 1).
2. Pretend that the pencil and lines separating the stripes are one dimensional and thus have a width of 0.
3. You can use any pencil length and stripe width but the easiest measurements are a width of 1 for the stripe and an length of 1/2 for the pencil.
4. The two factors to take into account are
A. How far into a particular stripe does the center of the pencil land?
B. The angle (from 0 to 90) that the pencil lands. (It’s easier to use radians…)
I can’t believe I just gave away so many hints.